This invention is directed to coated high speed flat stock material. More particularly, the invention pertains to metal strapping material having a powder coating thereon and a method for making same.
Strapping material is well known in the art. Such material is used for packaging, i.e. strapping goods, for example, to a pallet for transportation, storage and the like. Strapping materials, because they are used in such large quantities and are discarded after a single use, must be manufactured from relatively common materials in efficient, low-cost processes.
As will be recognized by those skilled in the art, often goods that are stored and or transported strapped to a base, such as a pallet, may be subjected to relatively severe environmental conditions. This is particularly true when the goods are transported overseas, such as by cargo ship. To this end, the severe environmental conditions may include exposure to saltwater and saltwater-laden air.
In addition, goods may be stored, in albeit less severe conditions, for prolonged periods of time. To this end, while the strapping may not be subjected to the severe conditions of saltwater-laden air, they may nevertheless be subjected to relatively high humidity environments.
It has been found that common steel strapping can corrode rapidly. That is, oxidation has been observed to begin almost immediately when the strapping is subjected to relatively high humidity conditions. Oxidation, i.e., rust can also compromise the integrity of the strap. In addition, it has been found that rust can stain or mar the appearance of the xe2x80x9cstrappedxe2x80x9d goods. This is particularly problematic with appearance sensitive products. Coatings have been used to prevent or retard corrosion of the strapping. One type of corrosion inhibiting coating is a water based coating much like a paint. Although these coatings work to an extent, it has been found that the process of coating the strapping material results in inconsistent coating or coverage and as such localized areas of corrosion can be readily observed. In addition, it has been found that with painted strapping, regardless of the coating thickness, corrosion of the strapping may nevertheless occur under prolonged or lengthy exposure conditions. It will be recognized by those skilled in the at that various qualities of liquid coatings are available, and that their corrosion resistance characteristics will vary. Nevertheless, there are drawbacks and limits to their performance characteristics.
Conventional wisdom provides that powder coatings be cured at temperatures of about 350xc2x0 F. to about 450xc2x0 F. for about 5 minutes to about 10 minutes. This precludes powder coatings for strap in that typical manufacturing lines speeds (about 180 to about 220 feet per minute) would require a curing oven hundreds of feet in length.
In addition, in the manufacture of steel strapping, the side edges of the strap are sharp and can create a personal hazard. Typically, the strap is conveyed around or over V-type pulleys which can abrasively remove the strap material at the edges resulting in sharp edges. Moreover, the protective function of a coating can be compromised by abrasive removal of the coating at the edges.
It has also been found that strap often requires an additional or subsequent application of an agent, such as wax, to increase the xe2x80x9cslipxe2x80x9d value of the finished material. A slip value is the force necessary to tension the strap when used in a strapping machine, when the strap is secured at one end and pulled or tensioned at an opposing end around a load. Slip values of less than about 15 Newton-meters are required for reliable tensioning of the strap. The use and operation of such a strapping machine is more fully disclosed in Bobren, U.S. Pat. No. 5,097,874, which patent is incorporated herein by reference.
Accordingly, there exists a need for a coating for strapping material that provides an effective barrier against corrosion. Desirably, such a coating is applied in a cost effective and efficient process that is compatible with existing metal strap manufacturing processes which require high manufacturing speeds (i.e., line speeds). Most desirably, such coating is applied resulting in a substantially uniform thickness of coating on the strapping material and, if desired, an over-coating of the strapping edges.
A corrosion-resistant strap is formed from an elongated steel bare strap material having width and a thickness and defining first and second sides and a pair of edge regions. A coating is applied and cured onto the bare strap material. The cured coating has a substantially consistent thickness at the first and second sides and at the edges. Optionally, the coating has a greater thickness at about the edge regions and on the first and second sides adjacent to the edge regions, defining a dog-bone profile.
For purposes of the present description and the claims that follow, reference will be made to bare strap, coated strap and cured strap. Bare strap is the base material prior to the application of the coating material. It is essentially the uncoated material that results from the xe2x80x9ctraditionalxe2x80x9d strap manufacturing process. Coated strap is the bare strap having the coating applied thereto, prior to curing or hardening. Last, cured strap refers to the strap having the coating applied thereto and cured or hardened.
As provided herein, a strap in accordance with the present invention has been shown to exhibit corrosion resistance characteristics in various simulated environments that are far superior to commercially available liquid coated strap. In some cases, these characteristics are more than ten-fold, and even twenty-fold increases over the known products.
Preferably, the coating is applied as a powder that is melted and cured onto the strap base material. A current powder is an epoxy material. Other contemplated powder materials include polyesters, urethanes, hybrids and the like.
A method for making the cured strap includes the steps of providing a bare strap having first and second sides and opposing edges. The strap is provided from a source, and is conveyed from the source to a coating apparatus. In that the coating operation or process can be fully integrated with the traditional strap manufacturing process, the xe2x80x9csourcexe2x80x9d can be the output of the strap making operation.
The bare strap or base material is directed through the coating apparatus. In a present method, the apparatus is oriented vertically so that the bare strap, coated strap and cured strap traverse upwardly through the apparatus. The apparatus can, however, be oriented horizontally or at any incline as well.
A powder is applied on the first side of the strap, which covers the first side and the opposing edges. The powder is likewise applied on the second side of the strap, covering the second side and the opposing edges. The method can include, when applying the powder to the first side, covering that portion of the second side immediately adjacent to the opposing edges, and when applying the powder to the second side, covering that portion of the first side immediately adjacent to the opposing edges. In this manner, there is a framing effect on the opposing side to that being covered. This results in a xe2x80x9cdog-bonexe2x80x9d profile of the coating on the bare strap.
The powder is melted to form a flowable material that coats the bare strap. The flowable material is cured on the strap, and the cured strap is cooled. The cured strap is then wound onto a storage member. When the vertical coating method is employed, the strap is preferably supported from only an uppermost point as it moves in the vertically upward direction. This prevents marring or damage to the newly applied coating.
The powder is applied using an electrostatic application process. Preferably, the powder is first applied to the first side of the strap and subsequently is applied to the second side of the strap. The coated strap is heated with the powder thereon as it moves through the apparatus.
In one method, an oven is provided for curing the coated strap. Preferably, heating occurs in a plurality of distinct heating zones within the oven. The method includes cooling the strap to a temperature of less than about 130xc2x0 F., and preferably about 70xc2x0 F. The powder is then applied to the strap first and second sides. The coated strap is conveyed through the oven to melt the powder and cure the melted powder on the strap forming the cured strap.
In an alternate method, the strap exits the xe2x80x9ctraditionalxe2x80x9d bare strap manufacturing process and powder is applied to the strap. When exiting the traditional manufacturing process, the strap is at a temperature of about 800xc2x0 F. which provides a latent heat in the strap. The latent heat in the strap is used to melt the powder to form the flowable material.
When the vertical method is used, the strap is directed vertically upward a sufficient distance to cool the strap. In a current method, the strap is conveyed upwardly a distance of about 100 feet. The strap is then redirected for windup onto the storage member. The strap is redirected using at least one crowned pulley per strap. Optionally, the strap can be cooled, such as by water spray. In the vertical method, this can be carried out while in the upward traverse, the downward traverse, or both. The water spray cooling can shorten the travel distance required to cool the cured strap.
As will be recognized by those skilled in the art, in the xe2x80x9ctraditionalxe2x80x9d strap manufacturing process, multiple straps are made in parallel. That is, the straps are cut or slit from a master roll of stock. To this end, the present coating processes are also carried out in parallel processing, coating and curing multiple straps at a time.
An apparatus for making a coated strap from bare strap in an in-line strap making apparatus includes a conveyance path. The path includes a spray region having electrostatic spray guns for applying a powdered coating to each of the first and second sides and the edge regions of the bare strap. A heating region is disposed subsequent to the spray region. The heating region has a sufficient length for melting the coating on the coated strap to cover the bare strap. The heating region can include an oven, such as an infra-red oven. Preferably, the oven has zones to provide melting and curing stages for the coated strap.
A cure region is disposed subsequent to the heating region. The cure region has a predetermined length sufficient for curing of the melted powdered coating. A cooling region is disposed subsequent to the cure region. The cooling region can include a liquid spray. The cooling region has a length sufficient for the cured strap to cool and harden the coating to prevent marring or damage during windup.
In a current apparatus, the conveyance path is vertically oriented from the spray region through the curing region. In this apparatus, the length of the cooling region is about 100 feet. However, it will be recognized by those skilled in the art that the length of the cooling region can vary depending upon the desired temperature of the cured strap prior to windup, and the particular cooling scheme (e.g., water spray) used. It has been determined that with a cooling spray, a cooling region as short as about 25 feet to about 30 can provide the necessary cooling.
These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.